Studies being conducted as part of an interagency agreement between NIEHS and FDA as part of the NIEHS AIDS effort are determining the relationship between decrements in immune cell phenotypes and susceptibility to infection or tumors. Studies conducted in the laboratory of Dr. James Weaver at FDA, using monoclonal antibodies (Moabs) against CD4 and CD8, to establish titration curves for the depletion of these cell subpopulations in B6C3F1 mice have been completed and published. At NIEHS, we are examining the effects of cell depletion in specific host resistance models. We are currently evaluating the effects of CD4+ cell depletion in the SA1 tumor model. It has been reported in the literature that clearance of this tumor is highly dependent on cell-mediated immune responses, although the relative contributions of CD4+ and CD8+ T cells remain to be addressed. We have also evaluated responses to influenza following depletion of specific immune cell populations and in COX deficient mice. Significant changes in body weight and temperature were observed in COX-1 null and wild type mice 1 day following influenza challenge, however the mice appeared to recover and clear the infection. In contrast, COX-2 null mice showed a lack of hypothermia and weight loss at early time points, but an accelerated response on day 3. Fifty percent of COX-2 null mice died between days 5 and 6 whereas there was no mortality in COX-1 null or wild type mice. Levels of the proinflammatory cytokines TNFalpha and IL-1beta and the anti-viral cytokine IFNgamma were dramatically reduced in bronchioalveolar lavage fluid from COX-2 deficient mice and the recruitment of neutrophils and macrophages to the airways was markedly attenuated these animals. We are currently assessing the relative viral burden in the tissues of these mice. Experimental animal data collected over the past 15 years using standardized testing panels has provided a database from which the sensitivity and predictability of a variety of tests commonly used for the screening of chemicals for immunotoxicity has been evaluated. These results have been used as guidelines for risk assessment in immunotoxicity and have been the basis for a number of regulatory activities. There has been considerable interest in the use of expanded histopathology, which focuses on examining the structural and architectural changes in lymphoid organ, as a primary screening test for immunotoxicity assessment. To determine the utility of this approach as a stand alone screen, a validation effort using data from the National Toxicology Program's immunotoxicology testing program was initiated. This study addresses the interlaboratory reproducibility of extended histopathology using a dataset of ten test chemicals and both negative and positive controls. We examined the consistency between pathologists with varied background in evaluating lesions in immune tissues and the sensitivity of the individual and combined histopathological endpoints to detect chemical effects and dose response. Agreement between pathologists was highest in the thymus, in particular with thymic cortical cellularity, and lower within all of the compartments examined in the spleen and lymph nodes. In addition, the analyses indicated that the ability to accurately identify histopathological change in lymphoid organs is dependent directly upon the experience/training that the individual possesses in immunohistology and the apparent severity of the specific lesion.